Reduced compression height dual gallery piston, piston assembly therewith and methods of construction thereof

ABSTRACT

A piston assembly, piston therefor and methods of construction are provided. The assembly includes a piston head and connecting rod operably coupled thereto via a wrist pin. The piston head has an upper crown with a combustion bowl and an undercrown surface. The lower crown includes axially aligned pin bores receiving the wrist pin. An upper wall of the lower crown has an oil inlet, an oil outlet and a concave, saddle bearing surface that bears against the wrist pin. A toroid-shaped outer cooling gallery is formed between wall portions of the upper and lower crowns, wherein the outer cooling gallery surrounds an inner cooling gallery. The connecting rod is fixed to the wrist pin for conjoint oscillation. The connecting rod has an oil passage in fluid communication with a through hole in the wrist pin to allow oil to flow therethrough into the inner cooling gallery via the oil inlet.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 62/098,181, filed Dec. 30, 2014, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to internal combustion engines, andmore particularly to pistons and their method of construction.

2. Related Art

Engine manufacturers are encountering increasing demands to improveengine efficiencies and performance, including, but not limited to,improving fuel economy, improving fuel combustion, reducing oilconsumption, increasing the exhaust temperature for subsequent use ofthe heat within the vehicle, increasing compression loads andtemperature within the cylinder bores, decreasing weight and makingengines more compact. In order to achieve these goals, the size andcompression height of the piston need to be reduced, however, theincreased temperature and compression loads within the combustionchamber require a robust piston to be maintained within workable thermaland loading limits. As such, although desirable to increase thetemperature and compression loads within the combustion chamber, thereis a tradeoff in that these increases limit the degree to which thecompression height, and thus, overall engine size, can be reduced.Further, the degree to which the weight of the engine can be reduced iscompromised in that the increased temperature and load demands placed onthe pistons require that they be made robust and of steel.

A piston constructed in accordance with this invention is able towithstand the modern performance demands discussed above, while at thesame time having a reduced compression height and reduced weight, aswill become apparent to those skilled in the art upon reading thedisclosure and viewing the drawings herein.

SUMMARY OF THE INVENTION

A piston constructed in accordance with one aspect of this invention isconstructed of steel, thereby providing the piston with enhancedstrength and durability to withstand increased compression loads withina cylinder bore, such as those seen in modern high performance engines.To further enhance the strength and rigidity of the piston, atoroid-shaped, outer cooling gallery can include a plurality ofcircumferentially spaced ribs extending along an undercrown surface,with the ribs extending radially between an outer ring belt region andan inner wall of the outer cooling gallery. Further, to enhance coolingthe piston, in addition to the outer cooling gallery, the piston furtherincludes a closed or substantially closed central inner cooling gallery,thereby allowing the piston to withstand elevated temperaturesencountered in modern, high performance engines. Further yet, due to thenovel configuration of the piston, the compression height (CH) andweight of the piston are able to be minimized, thereby allowing anengine in which the pistons are deployed to be made more compact andlightweight.

In accordance with one aspect of the invention, a piston assembly for aninternal combustion engine is provided. The piston assembly includes apiston head with a connecting rod operably coupled thereto via a wristpin. The piston head has upper and lower crowns, with the upper crownforming an upper combustion surface having a combustion bowl dependingtherein with an undercrown surface provided opposite the combustionbowl. The upper crown further includes a circumferentially continuousupper outer wall portion forming, at least in part, a ring belt region,and a circumferentially continuous upper inner wall portion dependingfrom the undercrown surface, wherein the upper inner wall portion isspaced radially inwardly from the upper outer wall portion in generallyconcentric relation therewith. The lower crown depends from the uppercrown to form a pair of laterally spaced pin bosses having axiallyaligned pin bores configured for receipt of a wrist pin. A pair ofdiametrically opposite skirt portions extend between the pin bosses, andan upper wall of the lower crown has a concave, saddle bearing surfaceformed therein, with the bearing surface extending between the pinbosses to provide a continuous bearing surface, in combination with thepin bores, that bears against the full length of the wrist pin. Theupper wall has at least one oil inlet and at least one oil outlet tofacilitate cooling the piston assembly. The lower crown has acircumferentially continuous lower outer wall portion and acircumferentially continuous lower inner wall portion, both extendingupwardly from the upper wall, wherein the annular upper and lower outerwall portions are fixed to one another and the annular upper and lowerinner wall portions are fixed to one another, thereby forming atoroid-shaped outer cooling gallery surrounding a central inner coolinggallery. The connecting rod has an end fixed to the wrist pin forconjoint oscillation therewith. The wrist pin has a through holeextending generally transversely to its length and the connecting rodhas an oil passage aligned for fluid communication with the through holeto allow oil to flow through the connecting rod, through the wrist pin,and through the oil inlet into the inner cooling gallery, whereupon theoil is free to flow outwardly from the inner cooling gallery through theat least one oil outlet.

In accordance with another aspect of the invention, the oil passage isconfigured for constant fluid communication with the through hole in thewrist pin and for intermittent fluid communication with the oil inlet.

In accordance with another aspect of the invention, the at least one oiloutlet is aligned to direct oil directly onto an outer surface of thewrist pin.

In accordance with another aspect of the invention, the at least one oiloutlet is aligned substantially tangent to an outer surface of the wristpin to facilitate lubricating the wrist pin.

In accordance with another aspect of the invention, the at least one oiloutlet can be provided to include a pair of oil outlets formed onopposite sides of the pin bore axis from one another, wherein each ofthe pair of oil outlets are configured to direct oil directly ontoopposite sides of the wrist pin to enhance lubrication of the wrist pinwithin the wrist pin bores.

In accordance with another aspect of the invention, the oil inlet isaligned along a central longitudinal axis along which the pistonreciprocates to facilitate providing intermittent alignment with thethrough opening in the wrist pin as the wrist pin oscillates.

In accordance with a further aspect of the invention, a plurality ofradially extending reinforcement ribs can be provided to extend alongthe undercrown surface within the outer cooling gallery, wherein thereinforcement ribs are spaced from the upper wall of the lower crown.

In accordance with another aspect of the invention, the outer coolinggallery can be formed with or without reinforcement ribs along theundercrown surface.

In accordance with a further aspect of the invention, the upper wallforms a substantially flat floor in the central cooling gallery tofacilitate reducing the compression height of the piston.

In accordance with a further aspect of the invention, the upper wallforms a substantially flat floor in the annular outer cooling gallery tofacilitate reducing the compression height of the piston.

In accordance with another aspect of the invention, the upper crown andthe lower crown can be joined to one another via friction welding orinduction welding.

In accordance with another aspect of the invention, the upper crown andthe lower crown can be formed as a monolithic piece of material in acasting process.

In accordance with another aspect of the invention, a piston for aninternal combustion engine is provided. The piston includes a pistonhead having upper and lower crowns, with the upper crown forming anupper combustion surface having a combustion bowl depending therein withan undercrown surface opposite the combustion bowl. The upper crownfurther includes a circumferentially continuous upper outer wall portionforming, at least in part, a ring belt region, and further includes acircumferentially continuous upper inner wall portion depending from theundercrown surface, wherein the upper inner wall portion is spacedradially inwardly from the upper outer wall portion in generallyconcentric relation therewith. The lower crown depends from the uppercrown to form a pair of laterally spaced pin bosses having axiallyaligned pin bores for receipt of a wrist pin. A pair of diametricallyopposite skirt portions extend between the pin bosses, and an upper wallof the lower crown has a concave, saddle bearing surface extendingbetween the pin bosses to provide a continuous bearing surface, incombination with the pin bores, that is configured to bear against thefull length of the wrist pin. The lower crown has a circumferentiallycontinuous, annular lower outer wall portion and a circumferentiallycontinuous, annular lower inner wall portion, both extending upwardlyfrom the upper wall of the lower crown. The upper and lower outerannular wall portions are fixed to one another and the upper and lowerinner annular wall portions are fixed to one another, thereby forming atoroid-shaped outer cooling gallery surrounding an inner coolinggallery. At least one oil inlet extends through the saddle bearingsurface into the inner cooling gallery such that is it substantiallycovered by the wrist pin during use. At least one oil outlet extendsthrough the upper wall in generally tangential relation with the saddlebearing surface such that oil from the inner cooling gallery ispermitted to flow outwardly from the central cooling gallery onto thewrist pin.

In accordance with a further aspect of the invention, the at least oneoil outlet includes a pair of oil outlets formed on opposite sides ofthe pin bore axis from one another to facilitate lubricating the saddlebearing surface.

In accordance with a further aspect of the invention, the at least oneoil inlet is aligned along a central longitudinal axis along which thepiston reciprocates.

In accordance with a further aspect of the invention, the piston canfurther include a plurality of reinforcement ribs extending radiallyalong the undercrown surface within the outer cooling gallery.

In accordance with a further aspect of the invention, the upper crownand the lower crown can be joined to one another by a weld joint.

In accordance with a further aspect of the invention, the upper crownand the lower crown can be formed as a monolithic piece of material in acasting process.

In accordance with a further aspect of the invention, the upper wallforms a substantially flat floor in the central cooling gallery tofacilitate reducing the compression height of the piston.

In accordance with a further aspect of the invention, the upper wallforms a substantially flat floor in the annular outer cooling gallery tofacilitate reducing the compression height of the piston.

In accordance with another aspect of the invention, a method ofconstructing a piston assembly for an internal combustion engine isprovided. The method includes forming a piston head and operablycoupling a connecting rod thereto via a wrist pin. The method includesforming the piston head having upper and lower crowns, with the uppercrown forming an upper combustion surface having a combustion bowldepending therein with an undercrown surface opposite the combustionbowl. The method includes forming the upper crown having acircumferentially continuous upper outer wall portion forming, at leastin part, a ring belt region, and a circumferentially continuous upperinner wall portion depending from the undercrown surface. The methodfurther includes forming the lower crown depending from the upper crownto form a pair of laterally spaced pin bosses having axially alignedconcave pin bores for receipt of a wrist pin. Further, forming a pair ofdiametrically opposite skirt portions extending between the pin bossesand an upper wall having a concave, saddle bearing surface extendingbetween the pin bosses to provide a continuous bearing surface, incombination with the pin bores, that bears against the full length ofthe wrist pin. Further yet, forming at least one oil inlet opening andat least one oil outlet opening through the saddle bearing surface.Further yet, forming the lower crown having a circumferentiallycontinuous lower outer wall portion and a circumferentially continuouslower inner wall portion, both extending upwardly from the upper wall,and fixing the upper and lower outer annular wall portions to oneanother and fixing the upper and lower inner annular wall portions toone another, thereby forming a toroid-shaped outer cooling gallerysurrounding an inner cooling gallery. Further, fixing an end of theconnecting rod to the wrist pin for conjoint oscillation therewith, andbringing an oil passage in the connecting rod into fluid communicationwith a through hole in the wrist pin, wherein the through hole in thewrist pin is brought into fluid communication with the at least one oilinlet in the upper wall during reciprocation of the wrist pin againstthe saddle bearing surface.

In accordance with another aspect of the invention, the method canfurther include configuring the oil passage in the connecting rod inconstant fluid communication with the through hole in the wrist pin andfor intermittent fluid communication with the at least one oil inlet inthe upper wall.

In accordance with another aspect of the invention, the method canfurther include aligning the at least one oil outlet to direct oildirectly onto an outer surface of the wrist pin.

In accordance with another aspect of the invention, the method canfurther include aligning the at least one oil outlet in substantiallytangent relation to an outer surface of the wrist pin.

In accordance with another aspect of the invention, the method canfurther include forming the at least one oil outlet as a pair of oiloutlets on opposite sides of the pin bore axis from one another.

In accordance with another aspect of the invention, the method canfurther include aligning the at least one oil inlet along a centrallongitudinal axis along which the piston reciprocates.

In accordance with another aspect of the invention, the method canfurther include forming a plurality of radially extending reinforcementribs along the undercrown surface within the outer cooling gallery,wherein the reinforcement ribs are spaced from the upper wall of thelower crown.

In accordance with another aspect of the invention, the method canfurther include forming the inner cooling gallery with or withoutreinforcement ribs along the undercrown surface.

In accordance with another aspect of the invention, the method canfurther include joining the upper crown and the lower crown to oneanother via friction welding or induction welding.

In accordance with another aspect of the invention, the method canfurther include forming the upper crown and the lower crown as amonolithic piece of material in a casting process.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the invention willbecome more readily appreciated when considered in connection with thefollowing detailed description of presently preferred embodiments andbest mode, appended claims and accompanying drawings, in which:

FIG. 1 is a sectioned side view taken generally along a pin bore axis ofa piston and connecting rod assembly constructed in accordance with oneaspect of the invention;

FIG. 2 is a sectioned side view taken generally transversely to the pinbore axis of the piston and connecting rod assembly of FIG. 1;

FIG. 3 is a sectioned side view of the piston of FIG. 1 taken generallyalong the pin bore axis;

FIG. 4 is a sectioned side view of the piston of FIG. 1 taken generallytransversely to the pin bore axis;

FIG. 5 is a sectioned bottom view of the piston of FIG. 1 takengenerally through the pin bore axis;

FIG. 6 is a view similar to FIG. 3 of a piston constructed in accordancewith another aspect of the invention;

FIG. 7 is a sectioned side view of the piston of FIG. 6 taken generallytransversely to the pin bore axis;

FIG. 8 is a view similar to FIG. 3 of a piston constructed in accordancewith yet another aspect of the invention;

FIG. 9 is a sectioned side view of the piston of FIG. 8 taken generallytransversely to the pin bore axis; and

FIG. 10 is a sectioned bottom view of the piston of FIG. 8 takengenerally through the pin bore axis.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

Referring in more detail to the drawings, FIG. 1 illustrates a pistonand connecting rod assembly, referred to hereafter as assembly 10,constructed in accordance with one presently preferred embodiment of theinvention, for reciprocating movement in a cylinder bore or chamber (notshown) of an internal combustion engine, such as a modern, compact, highperformance vehicle engine, such as a gas or diesel engine for example.The assembly 10 includes a piston 12, a connecting rod 14 and a wristpin 16. The connecting rod 14 is operably connected to the piston 12 viafixed attachment to the wrist pin 16 for conjoint oscillation with thewrist pin 16, such as via a threaded fastener or fasteners, by way ofexample and without limitation. The piston 12 has a body 18, shown inFIGS. 1-4 as being made of two separate pieces that are initiallyfabricated as separate parts and subsequently joined to one anotherwithin a head region across some form of a weld joint (i.e., inductionweld, friction weld, braze joint, charge carrier rays, laser,resistance, and the like). The two parts comprise a lower crown, alsoreferred to as bottom part 20, and an upper crown, also referred to astop part 22. Reference to “top”, “bottom”, “upper” and “lower” hereinare relative to the piston 12 being oriented along a verticallongitudinal central piston axis A along which the piston 12reciprocates in use. This is for convenience and is not to be limitingsince it is possible that the piston 12 may be installed and operate atan inclination other than purely vertical. The material (i.e., the steelalloy) used to construct the bottom and top parts 20, 22 may be the same(e.g., SAE 1040 grade) or different, depending on the requirements ofthe piston 12 in the intended engine application. The top part 22 may becast, may be machined from stock, may be sintered, forged or made by anynumber of processes. The bottom and top parts 20, 22, being constructedof steel, provide the piston 12 with enhanced strength and durability towithstand increased temperature and compression loads within thecylinder bore, and due to their novel configuration, along with thewrist pin 16 and connecting rod 14, minimize the weight and compressionheight CH (identified in FIG. 1 as the distance extending between a pinbore axis B and an upper combustion surface 24) of the piston 12,thereby allowing an engine in which the pistons 12 are deployed toachieve enhanced performance and attain a reduced weight and to be mademore compact.

A head region of the piston 12 has an annular top wall, also referred tothe upper combustion surface 24, with a central combustion bowl 26depending therein. The combustion bowl 26 is demarcated by a wall 28having a thickness (t, shown on FIG. 3), wherein the thickness (t)extends between the upper combustion surface 24 and an undercrownsurface 30 of the wall 28. The contour of the combustion bowl 26 isformed as being contoured to provide an upper apex or center peak 32that may lie coaxially along a central axis A of the piston 12 or may beradially offset relative to the piston central axis A. The contour ofthe combustion bowl wall 28 also provides an annular valley 34 whichsurrounds the peak 32, shown as being concentric in relation to the peak32 and forming the lowest portion of the combustion bowl 26. The wall 28of the combustion bowl 26 extends upwardly from the valley 34 to form acombustion bowl rim 33, which typically is where the greatest heat isgenerated during use.

The top part 22 of the piston 10 further includes a circumferentiallycontinuous, annular upper outer wall portion 36 forming, at least inpart, a ring belt 38, and further includes a circumferentiallycontinuous, annular upper inner wall portion, also referred to as upperinner ring 40, depending from the undercrown surface 30, wherein theupper inner ring 40 is spaced radially inwardly from the upper outerwall portion 36 in generally concentric relation therewith. The ringbelt 38 has a plurality of ring grooves in which piston rings (notshown) are received.

The bottom part 20 depends from the upper part 22 to form a pair oflaterally spaced pin bosses 42 having axially aligned concave pin bores44 aligned along the pin bore axis B that extends transversely to thecentral longitudinal axis A for receipt of the wrist pin 16. A pair ofdiametrically opposite skirt portions 46 extend between the pin bosses42, and a planar or substantially planar upper wall 48 of the lowercrown 20 has an arcuate, concave, saddle-shaped bearing surface, alsoreferred to as saddle bearing surface 50, extending between the pinbosses 42 to provide a continuous bearing surface in combination withthe pin bores 44, wherein the combined bearing surface bears against thefull length of the wrist pin 16. The upper wall 48 of the bottom part 20forms a planar or substantially planar upper surface 52, also referredto as floor 52 (FIG. 3), extending across an upper region of the bottompart 20. The bottom part 20 has a circumferentially continuous, annularlower outer wall portion 54 and a circumferentially continuous, annularlower inner wall portion 56, both extending upwardly from the uppersurface 52 in generally concentric relation with one another.

To facilitate cooling the piston 12 and lubricating the wrist pin 16,the upper wall 48 has at least one, and shown as a plurality ofopenings, shown as having at least one oil inlet, also referred to asport or opening 58 and at least one drainage opening, also referred toas oil outlet, port or opening 60. The oil inlet 58 is shown asextending along the central axis A, and the oil outlets 60 are shown asextending tangentially or substantially tangentially with an outersurface of the wrist pin 16 and the saddle bearing surface 50. The oilinlet 58 and oil outlets 60 are shown as being aligned with one acrossthe width of the wrist pin 16, though it is contemplated the oil outlets60 could be staggered or formed elsewhere along the length of the wristpin 16, preferably remaining tangential or substantially tangentialtherewith. Accordingly, oil flowing through the oil outlets 60 cool andlubricate the wrist pin 16 and the respective saddle bearing surface 50and pin bores 44.

An outer weld joint 62 that joins the separately made top and bottomparts 22, 20 of the piston 12 is formed between an annular first lowerjoining surface of the lower outer wall portion 54 of the bottom part 20and an upper first joining surface of the upper outer wall portion 36 ofthe top part 22. Further, an inner weld joint 64 is formed between asecond upper joining surface of the lower inner wall portion 56 of thebottom part 20 and a second upper joining surface of the upper innerwall portion 40 of the top part 22. It should be recognized that thefirst weld joint 62 can be formed in other locations within the ringbelt 38 by changing the respective lengths of the outer wall portion 36and lower outer wall portion 54. Further yet, as shown in an alternateembodiment in FIGS. 6 and 7, a piston 12′ constructed in accordance withanother aspect of the invention can be constructed in a casting processas a monolithic piece of material without having to join separate partsto one another. Substantially the same features as already discussedabout for the piston 12 are shown in FIGS. 6 and 7, and thus, no furtherdiscussion is believed necessary.

Upon fixing the bottom and top parts 20, 22 to one another, asubstantially closed, toroid-shaped outer cooling gallery 66 is formedadjacent the ring belt 38, wherein an oil inlet or opening 68 (FIGS. 2,4, and 5) extends through the upper wall 48 of the bottom part 20 intothe outer cooling gallery 66 to allow oil to be sprayed therein, such asvia an oil jet, by way of example and without limitation. Also, an oiloutlet or opening 70 extends through the upper wall 48 of the bottompart 20 on a generally diametrically opposite side of the piston, thoughslightly offset, to allow oil to exit the outer cooling gallery 66. Inaddition, an inner, central cooling gallery 72 is formed radiallyinwardly from the outer cooling gallery 66, wherein the inner coolinggallery 72 has a vaulted, generally conical upper surface formed by theundercrown surface 30 and a flat, planar floor formed by the uppersurface 52 of the bottom part upper wall 48, and can be formed as asubstantially closed cooling gallery, having only the inlet and outeropenings 58, 60. Otherwise, as shown in an alternate embodiment in FIGS.8-10, a piston 12″ constructed in accordance with another aspect of theinvention can be formed having a completely closed, sealed inner coolinggallery 72″. If constructed as a sealed cooling gallery, a coolant, suchas in the form of a metal-containing coolant composition, by way ofexample and without limitation, can be sealed within the cooling gallery72″. If provided as a metal-containing composition, the coolant materialmay be disposed into the cooling gallery 72″ in the form of a solid,liquid, or a mixture of solid particles and liquid. With the innercooling gallery 72″ containing a coolant medium sealed therein, the heatgenerated by the piston in the ring belt region does not have a thermaleffect on the coolant medium, and thus, the sealed coolant medium canimpart an increased cooling effect on the upper combustion surface 24,while the oil coolant circulating through the outer cooling gallery 66can more efficiently cool the ring belt region. Otherwise, the sealedinner cooling gallery 72″ could be void of any cooling medium, otherthan air, wherein it is to be recognized that the sealed air acts as aninsulator, thereby keeping the underlying saddle bearing surface and thewrist pin from overheating.

In accordance with a further aspect of the invention, the top part 22can be formed having radially extending ribs 74, wherein the ribs 74extend between the inner wall portion 40 and the outer wall portion 36.The ribs 74 can be provided in any desired number, and are preferablyspaced equidistantly from one another about the circumference of thepiston, thereby forming a “spoke-like” appearance extending radiallyoutwardly away from the axis A. In one embodiment, the ribs 74 arespaced 60 degrees from one another, thereby having 6 ribs, though otherarrangements and numbers or ribs are contemplated herein. The ribs 74are shown as extending downwardly from the undercrown surface 30 andstopping short of the upper surface 52 of the bottom part 20, therebyallowing oil to flow freely beneath the ribs 74. Accordingly, the ribs74 do not substantially impede the flow of the oil from circulatingthroughout the outer cooling gallery 66 between the inlet opening 68 andoutlet opening 70. The presence of the ribs 74, although relativelyshort and robust, greatly increases the compression load able to bewithstood by the piston, wherein testing has shown that the amount ofcompression load able to be withstood increases by about 40 percent ormore. In one exemplary test, the compression load of the pistonincreased from about 180 bar (without ribs 74) to about 250 bar (withribs 74).

The wrist pin 16 has at least one, and shown as a pair of threadedopenings 76 for receipt of fasteners (not shown) to fix the connectingrod 14 to the wrist pin 16. Accordingly, the wrist pin 16 and theconnecting rod 14 are fixed against relative movement with one another.The wrist pin 16 further includes opposite ends 78, 80 having pockets orrecesses 82 extending therein, thus, further reducing the weight of thepiston assembly 10 and moment of inertia thereof in use. Furthercontributing to a weight reduction is an oil hole, also referred to asthrough hole 84, extending along a widthwise direction midway betweenthe threaded openings 76 that allows oil to flow therethrough, thus,enhancing cooling and lubrication of the wrist pin 16 in use. Furtheryet, with the full length of the wrist pin 16 bearing against the pinbores 44 and the saddle bearing surface 50, the diameter of the wristpin 16 can be substantially reduced, as the wrist pin 16 is support overits full length and not in cantilevered fashion during a compression orexhaust stroke. Accordingly, the reduction in diameter of the wrist pin16 further reduces the weight of the assembly 10, while at the same timebeing able to withstand increased firing pressures relative toconventional cantilevered wrist pin designs.

The connecting rod 14 extends over a length between a large end (notshown) with configured for coupling to a journal of a crankshaft (notshown) and an opposite small end 86 that is configured for fixedattachment to the wrist pin 16. The end 86 has a flange with a pair ofthrough openings 88 configured for alignment with the threaded openings76 in the wrist pin 16. With the through openings 88 aligned with thethreaded openings 76, the fasteners are inserted through the throughopenings 88 and threaded into the threaded openings 76 to fix theconnecting rod 14 to the wrist pin 16. The connecting rod 14 has an oilpassage 90 extending over its length, and to avoid interfering with thelarge end (not shown), the oil passage 90 is inclined relative alongitudinal central axis of the connecting rod 14, and thus, althoughbeing configured for fluid communication with the through opening 84 ofthe wrist pin 16, the oil passage 90 and through opening 84 are inclinedrelative to one another. Accordingly, the oil passage 90 is aligned withthe through hole 84 in the wrist pin 16, and as such, allows oil to flowfreely through the connecting rod 14, through the wrist pin 16, throughthe inlet opening 58 into the central cooling gallery 72, whereupon theoil can drain freely from the central cooling gallery 72 through theoutlet openings 60. As such, the wrist pin 16 is cooled and lubricated,while also cooling the combustion bowl wall 28 via the central coolinggallery 72.

The piston 10 is adapted for use in light, modern, high performancevehicle diesel engine applications. While made of steel, the piston 10,by its thin-walled design, is as light, if not lighter, than itsaluminum counterparts when taking into account the mass of the aluminumpiston and the associated insert pin bore bushings, etc used in aluminumpiston assemblies. The steel piston 10 also has a significantly smallercompression height CH. The comparable weight and smaller CH allows theengine to be made smaller and more compact. The greatly reduced CH ismade possible by synergistic features, including the reduced diameterwrist pin 16, the raised location of the saddle-shaped uppermost wristpin saddle bearing surface 50 against which the wrist pin 16 oscillates,the relatively low profile configuration of the cooling galleries 66,72, due in part to the flat or substantially flat floor surface 52, andthe fixation of the connecting rod end 86 directly to the wrist pin 16without having to wrap around the wrist pin 16.

As a result of the reduced CH and overall envelop of the piston 12, thefull length of the connecting rod 14 can be extended by the same orabout the same length as the reduction in CH. However, if desired, thesame center-to-center relation of a standard connecting rod can beattained by reducing the crank-to-deck face height, thereby reducing theengine block weight and envelop of the engine. Otherwise, the length ofthe connecting rod 14 can be extended, which in turn, results in areduced angularity of oscillation over a full stroke, and thus, reducesthe thrust load imparted on the skirt panels 46. This also results in areduction of impact losses stemming from the translation of the pistonfrom one side of the liner to the other, thereby reducing linercavitation. Further, with the length of the connecting rod beingincreased, an increase in thermal efficiency is attained due to theadded residence time per crank angle spent by the piston 12 close totop-dead-center (TDC) during combustion, thereby increasing theefficiency of conversion into brake power.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A piston assembly for an internal combustionengine, comprising: a piston head having an upper crown and a lowercrown joined together by an annular outer wall and an annular innerwall, said upper crown having an upper combustion surface and acombustion bowl depending therein with an undercrown surface providedopposite said combustion bowl, said lower crown having an upper wall anda pair of laterally spaced pin bosses depending from said upper wall,said pin bosses having pin bores aligned axially with one another alonga pin bore axis, a concave bearing surface formed in said upper wall andextending between said pin bosses to form a continuous bearing surfacewith said pin bores, a toroid-shaped outer cooling gallery boundedbetween said annular outer wall and said annular inner wall and an innercooling gallery bounded by said annular inner wall, said upper wallhaving at least one oil inlet and at least one oil outlet extending intosaid inner cooling gallery; a wrist pin having a through hole; and aconnecting rod having an end fixed to said wrist pin for conjointoscillation therewith, said connecting rod having an oil passage alignedfor fluid communication with said through hole in said wrist pin andwith said oil inlet to allow oil to flow through the connecting rod,through the wrist pin, and through the oil inlet into said inner coolinggallery and outwardly from said inner cooling gallery through said atleast one oil outlet.
 2. The piston assembly of claim 1 wherein said oilpassage is configured for constant fluid communication with said throughhole and for intermittent fluid communication with said at least one oilinlet.
 3. The piston assembly of claim 1 wherein said at least one oiloutlet is aligned to direct oil directly onto an outer surface of saidwrist pin.
 4. The piston assembly of claim 3 wherein said at least oneoil outlet is aligned substantially tangent to an outer surface of saidwrist pin.
 5. The piston assembly of claim 4 wherein said at least oneoil outlet includes a pair of oil outlets formed on opposite sides ofsaid pin bore axis from one another.
 6. The piston assembly of claim 1wherein said one oil inlet is aligned along a central longitudinal axisalong which said piston reciprocates.
 7. The piston assembly of claim 1further including a plurality of reinforcement ribs extending radiallyalong said undercrown surface within said outer cooling gallery.
 8. Thepiston assembly of claim 1 wherein said upper crown and said lower crownare joined to one another by a weld joint.
 9. The piston assembly ofclaim 1 wherein said upper crown and said lower crown are formed as amonolithic piece of material in a casting process.
 10. The pistonassembly of claim 1 wherein said upper wall forms a substantially flatfloor in said inner cooling gallery.
 11. The piston assembly of claim 10wherein said upper wall forms a substantially flat floor in said outercooling gallery.
 12. The piston assembly of claim 1 wherein said throughhole extends along a first longitudinal axis and said through passageextends along a second longitudinal axis, said first longitudinal axisbeing inclined relative to said second longitudinal axis.
 13. A pistonfor an internal combustion engine, comprising: a piston head havingupper and lower crowns, said upper crown including an upper combustionsurface having a combustion bowl depending therein with an undercrownsurface opposite said combustion bowl, said upper crown furtherincluding an annular upper outer wall portion and an annular upper innerwall portion depending from said undercrown surface, said upper innerwall portion being spaced radially inwardly from said upper outer wallportion; and said lower crown depending from said upper crown andincluding a pair of laterally spaced pin bosses having axially alignedpin bores for receipt of a wrist pin and a pair of diametricallyopposite skirt portions extending between said pin bosses, said lowercrown including an upper wall having a concave, saddle bearing surfaceextending between said pin bosses to provide a continuous bearingsurface, in combination with said pin bores, said lower crown includingan annular lower outer wall portion and an annular lower inner wallportion, said lower annular outer and inner wall portions extendingupwardly from said upper wall, said upper and lower annular outer wallportions being fixed to one another and said upper and lower annularinner wall portions being fixed to one another and forming a outercooling gallery surrounding an inner cooling gallery; and including atleast one oil inlet extending through said saddle bearing surface intosaid inner cooling gallery, and wherein said outer oil gallery includesat least one oil inlet and at least one oil outlet both of which arespaced laterally from and out of communication with said saddle bearingsurface and in open communication with an interior space inboard of saidskirt portions.
 14. The piston of claim 13 further including at leastone oil outlet extending through said upper wall in generally tangentialrelation with said saddle bearing surface to allow oil to flow outwardlyfrom said inner cooling gallery onto the wrist pin.
 15. The piston ofclaim 14 wherein said at least one oil outlet includes a pair of oiloutlets formed on opposite sides of said pin bore axis from one another.16. The piston of claim 14 wherein said at least one oil inlet isaligned along a central longitudinal axis along which the pistonreciprocates.
 17. The piston of claim 13 further including a pluralityof reinforcement ribs extending radially along said undercrown surfacewithin said outer cooling gallery.
 18. The piston of claim 13 whereinsaid upper crown and said lower crown are joined to one another by aweld joint.
 19. The piston of claim 13 wherein said upper crown and saidlower crown are formed as a monolithic piece of material in a castingprocess.
 20. The piston of claim 13 wherein said upper wall forms asubstantially flat floor in said inner cooling gallery.
 21. The pistonof claim 20 wherein said upper wall forms a substantially flat floor insaid outer cooling gallery.
 22. The piston of claim 13 wherein saidinner cooling gallery is sealed.